1. Field of the Invention
The present invention generally relates implantable medical systems and, in particular, to systems, devices and methods for detecting incipient edema.
2. Description of the Prior Art
Pulmonary edema is often associated with heart failure.
In a healthy patient, the small alveoli in the lungs are filled with air. Close to the alveoli is a thin interstitial space, filled with interstitial fluid, which is different from blood. For example, the interstitial fluid contain no red blood cells. The interstitial space is also in close connection with capillaries filled with blood. Oxygen in the alveoli is diffusing into the interstitial space and then into the blood in the capillaries.
Development of pulmonary edema occurs gradually. For example, pulmonary edema may be a consequence of a heart failure. When the heart acquires a heart failure, the left arterial pressure and thereby the pressure in the capillaries will increase during a first phase. In this phase, no edema is present. In a second phase, the interstitial space between the alveoli and the capillaries is filled with more fluid, i.e., a swelling of the interstitial space occurs. The volume of the interstitial space increases with about two deciliters. In the following third, exacerbated phase, when the left arterial pressure has increased above 20, 25 mmHg, fluid enters the alveoli. Fluid in the alveoli diminishes the degree of oxygenation of the blood which weakens the heart and causes peripheral vasodilatation. The peripheral vasodilatation increases venous return from the peripheral circulation, which further increases the damming of blood in the alveoli, and thus, further diminishes the degree of oxygenation of the blood. This course of events may eventually lead to death of the patient. Pulmonary edema may develop so rapidly that death can occur within 20 minutes to an hour. Consequently, early detection of incipient edema is critical.
Pulmonary edema may be detected by using transthoracic impedance and through impedance measured between two implanted electrodes, covering one of the lungs. Edema has also been detected by listening to lung sounds.
Further, in U.S. Pat. No. 6,332,091, detection of pulmonary edema using infrared light is described. In a non-invasive method, a lung is exposed to infrared light and the reflected radiation scattered by the lung as a spectral response to the presence of water in the lung is measured. The reflected radiation is compared with calibrated values to evaluate an occurrence of pulmonary edema. The non-invasive method disclosed in U.S. Pat. No. 6,332,091 requires the use of external equipment, consequently pulmonary edema may only be detected in a patient at locations where such equipment is present.
In U.S. Pat. No. 7,010,337, a sensor for measuring transmission or reflection of light by the blood is described. The sensor is placed adjacent the aorta of a patient to evaluate the oxygen saturation of the blood passing in the aorta. Detection of edema is not discussed in U.S. Pat. No. 7,010,337.
In U.S. Pat. No. 6,409,675, a implantable monitor with one or more sensors configured for extravascular placement is described. The extravascular sensors include sensors for vascular plethysmography, heart and lung sounds, thoracic impedance, and EKG. For example, optical sensors adapted to determine arterial blood oxygen and arteriolar volume, and sound sensors adapted to detect pulmonary edema are disclosed. As alternate embodiments, detection of pulmonary edema utilizing thoracic impedance, ultrasound or analysis of Cheyne-Stokes respiration are disclosed.
Consequently, there remains a need within the art for an implantable medical system and a method using such system that are capable of detecting the occurrence of edema at an early stage, i.e. incipient edema.